A bressummer, or breastsummer, in timber-building, is a beam

Beam (structure)

A beam is a horizontal structural element that is capable of withstanding load primarily by resisting bending. The bending force induced into the material of the beam as a result of the external loads, own weight, span and external reactions to these loads is called a bending moment.- Overview...

 in the outward part of the building, and the middle floors, (not in the garrets or ground floors) into which the girder

Girder

A girder is a support beam used in construction. Girders often have an I-beam cross section for strength, but may also have a box shape, Z shape or other forms. Girder is the term used to denote the main horizontal support of a structure which supports smaller beams...

s are framed. In the inner parts of a building, such beams are called "summers".

It is part of the timber frame

Timber framing

Timber framing , or half-timbering, also called in North America "post-and-beam" construction, is the method of creating structures using heavy squared off and carefully fitted and joined timbers with joints secured by large wooden pegs . It is commonplace in large barns...

 construction in the over-hanging upper story in jettying

Jettying

Jettying is a building technique used in medieval timber frame buildings in which an upper floor projects beyond the dimensions of the floor below. This has the advantage of increasing the available space in the building without obstructing the street...

In addition, breastsummer (but not bressummer) can also be used to describe a dark shade of green. The name of the color comes from the shade of green of the leaves on the trees that were originally used to build bressummers.